Electroless copper plating

ABSTRACT

Electroless copper plating solutions can be stabilized against autodecomposition by the addition of minute quantities of organothiophosphate compounds. A secondary stabilizer or brightener of the propargyl-type gives added improvement.

O United States Patent 1 1111 3,790,392 Gilano Feb. 5, 1974 ELECTROLESS COPPER PLATING' 3,635,758 1/1972 Schneble eta]. 106/1 Inventor: N. Gilano, Fullerton, P [73] Assignee: Dynachem Corporation, Santa Fe 1,196,464 7/1965 Germany 106/1 Springs, Calif. 1

[22] Filed: Jan. 17, 1972 Primary Examiner-Lorenzo B. Hayes [21] p No 218 459 Attorney, Agent, or Firm-Bert J. Lewen [52] US. Cl. 106/1, 117/47 A, 117/130 E ABSTRACT i zld l f IIIIIIIIII 3 2 Electfoless copper plating solutions can be stabilized against autodecompoition by the addition of minute quantities of organothiophosphate compounds. A sec- [56] References Cited ondary stabilizer or brightener of the propargyl-type dd d t. UNITED STATES PATENTS gwes a e mprovemen 3,457,089 7/1969 l8 Clain s, No Drawings Shipley et al. 106/1 ELECTROLESS COPPER PLATING DESCRIPTION OF THE INVENTION Electroless copper plating solutions which employ alkaline formaldehyde as the reducing agent for cupric ions are autocatalytic and therefore frequently are unstable, i.e., they have a tendency to plate-out prematurely. Many methods have been proposed to minimize the autodecomposition of electroless copper baths. The use of strong chelating agents, such as ethylenediaminetetraacetic acid (EDTA), for example, is known to be helpful in retarding the rate of autodecomposition. (See U.S. Pat. No. 3,119,709). Chelation, however, does not give completely satisfactory stabilization, and, in many cases, so adversely affects the rate of metal deposition as to make the plating process commercially useless.

It has also been recognized that the cuprous ion is extremely active in promoting the autodecomposition of electroless copper plating solutions. To reduce the cuprous ion concentration it has been proposed to bubble air or oxygen through the electroless plating solution. (See U.S. Pat. No. 2,938,805). This practice converts the cuprous ion to cupric ion and is commonly used in the art. This method, when used as the sole means of bath stabilization, has two major drawbacks. Firstly, the deposits which result are usually dark and nonmetallic in appearance, probably due to an outer layer of cupric oxide; and, secondly, a large amount of formaldehyde is volatilized by the oxygen passing through the solution, making control of the chemical balance more difficult.

The use of minute amounts of various chemicals capable of complexing cuprous ions is also well-known in the art for increasing the stability of electroless copper solutions. Typical of such additives are cyanides, nitriles, inorganic sulfides, and various organic divalent sulfur compounds. (See U.S. Pat. Nos. 3,095,309 and 3,36 l ,5 80). In general, these additives too have several disadvantages. In most cases they only moderately extend the useful life or operating parameters of the electroless copper solutions; and, in cases where the stability is greatly enhanced, the rate or quality of metal decomposition is generally slowed. Therefore, some compromise between stability of the electroless plating solution and the quality or quantity of the metal deposit is usually necessary.

In accordance with the instant invention, it has been discovered that the conventional electroless copper solution can be stabilized over a wide range of temperatures for extremely long periods with no sacrifice in the quality or color of the metal deposit or the rate of deposit. The electroless copper baths of the invention are used continuously, with replenishing of the constituents lost by chemical reaction or drag-out, at high efficiency.

Additionally, the baths as stabilized by this invention will tolerate repeated heating and cooling procedures and will generally operate efficiently at ambient temperatures. The resulting copper films are bright pink and composed of pure copper metal. They are completely devoid of dark and grainy areas of copper oxides commonly seen in previously known films.

The stabilizer of the instant invention may be characterized by the following general formula:

wherein R and R' are the same or different and are a lower alkyl group having from 1 to 12 carbon atoms, preferably from 1 to 4 carbon atoms or an aryl group, such as a phenyl or a naphthyl group, or a substituted alkyl or aryl group. In all events'the R-X and R-Y bonds should be stable in the alkaline bath and resist hydrolysis. X, Y and Z may be oxygen and sulfur, most preferably X and Y are oxygen. R may represent a wide variety of chemical groups. The main criteria in selecting R is that it, together with the Z, form an acidic group which will slowly hydrolyze in the alkaline plating bath so as to form an R (X)R(Y)P(S z) moiety. Any

structure where the carbon attached to the Z is bonded to a moiety other than an alkyl group is suitable. More specifically, R may be hydrogen, a substituted alkyl or aryl group wherein the substitution is a halogen, e.g., chloride, bromide or iodide; a hydroxyl; an amino or lower alkyl or alkanol amino; nitro; carbalkoxy; alkylthio; alkoxy; or aroxy group. The alkyl groups may have 1 to 12 carbon atoms, preferably, from I to 4. The aryl group may be phenyl or naphthyl. Such groups as: a para-nitrophenyl group, an ethylthioethyl group, a N-methylcarbamoylmethyl group, a trichloroethyl group, a l,2-di(ethoxycarbonyl) ethylthio group. In no event, however, shall more than one of the three R groups be hydrogen.

Representative compounds include: diethyl pnitrophenyl thionophosphate (Parathion); dimethyl S-2-ethylthioethyl thiolophosphate; monomethylamide of O,O-dimethyldithiophosphoryl acetic acid; diethyl ester of 0,0-dimethyldithiophosphoryl succinic acid (Malathion); and diethyl 2-isopropyl-4-methylpyrimid- 6-yl thionophosphate.

Most preferred of these compounds are the diethyl p-nitrophenylthionophosphate and diethyl ester of 0,0-dimethyl dithiophosphoryl succinic acid, known by the trivial names Parathion and Malathion, respectively.

Based on the moles of cupric salt employed, from 0.0001 to 0.001 moles of the organic phosphate stabilizer is added to the electrolessplating solution. Preferably, from 0.0002 to 0.0004 molesare employed.

In order to further enhance the stability of the electroless plating bath and the characteristics of the plated film, it is a preferred embodiment of the invention to add a propargyl-type compound to the solution. This compound may be represented by the generic formula:

In the above formula, the X may be the bivalent oxygen, carbonyl, sulfur, sulfonyl, sulfoxide or imino or a trivalent nitrogen; n is an integer from I to 3. Where X is bivalent, R" may be an alkyl group having from I to 12 carbon atoms, preferably from 1 to 6; a phenyl or naphthyl group. Where X is the trivalent nitrogen, R" may represent two or more of the aforesaid groups, or may form a ring with carbon alone or carbon and oxygen jointly with the nitrogen group. Most preferably, the X in the above formula is a nitrogen group and the R" and the N together represent a phthalimide or a propargyloxy phthalimide. Examples of other compoundswhich may be employed include: N- propargylmaleimide; N-propargylsuccinimide; N-alkyl- N-propurgylamides; N,N-dialkyl-N-propargylamines; aryl and alkyl propargyl ethers; aryl and alkyl propargyl thioethers; aryl and alkyl .propargyl ketones; and aryl and alkyl propargyl sulfones.

The amount of propargyl-type compound which may be used is preferably expressed in terms of the cupric salt in a liter of the electroless copper plating bath. Generally from 0.0001 to 0.001 moles are-employed, preferably from 0.0002 to 0.0004.

The electroless copper plating solutions of the present invention are alkaline aqueous solutions containing a source of cupric ions, at least one complexing agent for cupric ions, and an active reducing agent. The alkalinity can typically be provided by sodium or potassium hydroxides, carbonates, or phosphates, although not limited to these bases. For the purpose of this invention, the preferred alkali is a mixture of an alkali metal hydroxide and carbonate. .This mixture is economical and allows a facile control of pH. The sodium salts are generally preferred due to their low cost.

SuitaBlsources drastic ion are water-samsiaza per salts such as cupric sulfate, cupric nitrate, cupric' s uitab'le reducing agents are formaldehydeand formaldehyde sources including aqueous formaldehyde, paraformaldehyde, and derivatives thereof. For the purpose of this invention, aqueous formaldehyde is the preferred reducing agent due to its low cost, availability, and convenience of use.

' Those skilled in the art will also recognize that other additives such as surfactants may be utilized to fully enhance the performance of an electroless copper plating solution.

'- Broadly speaking, in concentration per liter of plat-= ing solution, the copper sulfate may be present from 0.002 to 0.15 moles, preferably from 0.002 to 0.04

moles. Sufficient alkali should be present to give a pH- from 10.5 to 14, preferably a pH of 13.0 to 13.5. The formaldehyde, or equivalent, may be present from 0.06 to 1.3 moles, preferably from 0.25 to 0.50 moles. The complexing agent should be 1 to 4 times the moles of copper present, and preferably approximately 2 to 2.5 times to the cupric salt present.

As noted previously, the electroless copper plating solutions of the invention are stable for an extended perind of time at ambient temperatures. Furthermore,

they can withstand temperatures up to 12()F., and

even in some cases to boiling, without deleterious c1- fects.

The electroless plating solutions of this invention are preferably maintained at a specific gravity of 1.04 1.05 and a temperature of F. Under these conditi orisf a deposition rate of approximately one millionth of an inch-per minute can be achieved. At elevated temperature, deposition rates are increased.

In the typical mode of operation, the electroless copper plating solution is held in plastic or plastic-lined metal tanks at 70-- l0OF., preferably with mechanical agitation. The pieces to be plated are cleaned and sensitized, if necessary, by methods wellknown to those skilled in the art. Immersion of the object to be plated for 1030 minutes is generally sufficient to produce the desired plating thickness. Subsequent deposition of ad-' ditional metal plate by electrolytic means is then easily accomplished, if desired.

The surface to be plated must be free of grease and other contaminating material. Where a non-metallic surface is to be plated, the surface areas to receive the deposit should first be treated, as in conventional processes, with conventional sensitizing and seeding solutions, such as stannous chloride (SnC1 followed by treatment with a dilute solution of palladium chloride (PdCI Where a metal surface, such as stainless steel, is to be treated, it should be. degreased, and then treated with acid, such as hydrochloric or phosphoric acid to free the surface of any oxide. If the electroless deposit is to be made on a plastic or ceramic base which is impregnated with cuprous oxide (Cu,O), the cleaned base is immersed in the electroless plating bath and allowed to remain until the deposit is sufficiently thick.

To further illustrate the invention, attention is directed toward the followingexamples:

EXAMPLE I In accordance with the invention, seven electroless copper plating solutions are'prepared. Formulation of the solutions is as follows: To approximately one-half liter of water is added, in the order shown in the table, the several compounds named.'Prior to addition, the stabilizers are solvated with a cosolvent, such as a glycol ether, as weill be-readily understood by one skilled in the art. After all of the components are added, water sufficient to make one liter is added. Each of the solutions contains 9.25 grams of CuSO '5 H 0; 16 grams of NaOH; 5 grams of Na CO and 30 grams of 37% formaldehyde.

The following table lists, in grams the other components present in the electroless copper plating solutions of the invention:

"" TABLE A Compound Solution Number 1 2 4 5 6 Rochelle Salts 16.6 16.6 33.2 33.2 Na,EDTA-2H,O 16.6 16.6 33.2 33.2

, Malathion 0.005 I 0.005 0.0025 0.005 0.0025 0.0025

N-propargyl-phthalimide 0.0025 0.0025

Accelerated stability tests are carried out by sealing each of the above solutions in glass vials and storing at 130F; for up to 12 days. The following table describes the percent loss of cupric ion at various periods of time 6 EXAMPLE It In order to show the use of other stabilizers within the scope of the invention, additional solutions are pre during storage. 5 pared. These solutions are essentially the same as Solu- TABLE B Time at 130F. Percent Loss of Cupric lon Solution Number 1 2 3 4 5 6 Control* 1 hr. nd nd nd tr nd nd 20 2 hrs nd nd l l0 nd nd 50 3 hrs nd nd tr tr 75 4 hrs nd nd 50 20 1O hrs nd tr 10 100 6 hrs tr 30 l5 72 hrs. l5 100 100 12 days 50 50 50 40.

*Same as Solution No. 2 without the Malathion.

tr trace. nd not detectable.

The above table shows the markedly improved stability of the solutions of the invention as compared to the control. A solution identical to Solution No. 1, except that the Malathion was omitted, has essentially the same stability as the control, though 0.005 grams of the phthalimide is present. In order to demonstrate theefficacy of the copper plating solutions of the invention, each of the seven solutions are used to plate epoxy plastic panels. The panels are scrubbed and sensitized according to procedures well-known in the art. The sensitized panels are then immersed into beakers containing each of the above solutions at 75F. and pH 13.3 for ten minutes. The following table shows the electroless copper plating thickness observed for each of the solutions:

Ineach case, except for the control, the plated material contained a copper plating approximately 10 millionths of an inch thick. This is equivalent to a plating rate of one millionth of an inch per minute. The copper plate is of excellent quality, pink in color, and free from impurities. A particularly good'quality plate is obtained where the secondary stabilizer is added.

While this plating rate with thecontrol is somewhat faster, the copper plate obtained is not of high quality and shows the presence of decomposition products which cause dark and grayish areas in the plate.

The aforesaid comparison clearly shows that the electroless copper plating solutions of theinvention are stable, yields a high grade copper plate, and have a satisfactory deposition rate. One skilled in the art will readily understand that, for a particular plating bath, substrate, and plating conditions, the optimum amount of the stabilizers may vary. By routine experiment, one can achieve the best balance of deposition rate, stability and plating quality.

tion N6. 5 described in Example l, except that the stabilizers are 0.005 grams of various dialkyl mercaptothionophosphates and are used in place of the Malathion. Using the same tests as shown in Example I the following stability and plating rates are obtained:

TABLE 1) Loss after 72 hr. at 130F.

Stabilizer Plating Thickness (millionths) after 10 min.

4.1 Diethylmercaptothionophosphute 14.1 Di-m'propylmercaptothionophosphate Dimethylmercaptothionophosphate In all examples above, the copper plate is of excellent quality, pink in color, and free from impurities.

EXAMPLE lIl are similar to those obtained for Solution No. 1

The stabilized electroless copper plating solutions described in the above examples are stable for extended periods of time at ambient temperatures and at elevated temperatures up to F. when compared to control solutions without the stabilizers of this invention. Electroless plating solutions containing the stabilizers of this invention can, in some cases, be brought to the boil without deleterious effects.

Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. In an alkaline electroless copper plating bath having a pH in the range 10.5 to 14, and comprising water, a water soluble copper salt, a complexing agent for cupric ion, and formaldehyde, the improvement of maintaining in the bath from 0.0001 to 0.001 mole, per mole of cupric ion, of a primary stabilizer having the following general formula:

wherein R and R' are the same or different, and are an alkyl group having from 1 to 12 carbon atoms, a phenyl or naphthyl group; X, Y and Z are oxygen or sulfur; and R is hydrogen, a substituted alkyl or a substituted aryl group, wherein the substitution may be halo, a hydroxyl, an amino or lower alkylor alkanol-amino, a nitro, a carbalkoxy, alkylthio, alkoxy, or aryloxy group; wherein the alkyl groups have from.1 to 12 carbon atoms and the aryl groups are phenyl or naphthyl.

2. The electroless plating bath of claim 1 wherein from 0.0001 to 0.001 mole, per mole of cupric ion, of a secondary stabilizer having the general formula:

R"X(CH ),,C E CH wherein R" is an alkyl group having 1 to 12 carbon atoms, a phenyl or naphthyl group, or a substituted phenyl or naphthyl group; X is a thio, a sulfonyl, a sulfoxide, oxy, carbonyl orimino group; n is l, 2 or 3; or R"X is an imide or a heterocyclic ring composed of nitrogen and carbon with or without oxygen group, is also maintained in the bath.

3. The electroless copper plating bath of claim 1 wherein X and Y are oxygen and R is an alkyl group of l to 4 carbon atoms.

4. The electroless copper plating bath of claim 1 wherein the primary stabilizer is diethyl ester of 0,0-dimethyldithiophosphoryl succinic acid or diethyl p-nitrophenyl thionophosphate.

5. The electroless copper plating bath of claim 1 wherein ZR is a mercapto group.

6. The electroless copper plating bath of claim 1 wherein the primary stabilizer is a dialkylmercaptothionophosphate.

7. The electroless copper plating bath of claim 6 wherein the dialkyl group is dimethyl, diethyl or di-npropyl.

8. The electroless copper plating bath of claim 2 wherein n is l and RX' is a phthalimide group.

9. The electroless copper plating bath of claim 2 wherein the bath contains diethyl ester of 0,0-dimethyl'dithiophosphoryl succinic acid and N- propoargylphthalimide or N-propargyloxyphthalimide.

10. In a process of electroless copper plating which includes immersing a receptive surface to be plated in an alkaline aqueous bath having a pH in the range 10.5 to 14, and comprising water, a water soluble copper salt, a complexing agent for cupric ion, and formaldehyde, the improvement of maintaining, in the bath 0.0001 to 0.001 mole, per mole of cupric ion, ofa primary stabilizer having the general formula:

wherein R and R" are the same or different, and are an alkyl group having from 1 to 12 carbon atoms, a phenyl or a naphthyl group;X, Y and Z are oxygen or sulfur; and R is hydrogen, a substituted alkyl or a substituted aryl group, wherein the substitution may be halo, a hydroxyl, an amino or lower alkylor alkanolamino, a nitro, a carbalkoxy, alkylthio, alkoxy, or aryloxy group; wherein the alkyl groups have from 1 to 12 carbon atoms and the aryl groups are phenyl or naphthyl.

11. The process ofclaim l0fwherein from 0.0001 to 0.001 mole, per mole of cupric ion, of a secondary stabilizer having the general formula:

wherein, R" is an alkyl group having l to 12 carbon atoms, a phenyl ora naphthyl group, or a substituted phenyl or naphthyl group; X is a thio, a sulfonyl, a sulfoxide, oxy, carbonyl or imino group, n is 1, 2 or 3; or R"R' is an imide or a heterocyclic ring composed of nitrogen and carbon with or without oxygen group, is also maintained in the bath.

12. The process of claim 10 wherein X and Y are oxygen and R is an alkyl group of 1 to 4 carbon atoms.

13. The process of claim 10 wherein the primary sta- 18. The process of claim 11 wherein the bath contains small amounts of diethyl ester of 0,0-dimethyldithiophosphoryl succinic acid and N- propargylphthalimide or N-propargyloxyphthalimide.

' =1: it: w a: a

, UNITED STATES PATENT OFF-ICE CERTIFICATE OF CORRECTION Patent No. 3: 9 :39 Dated February 5, 197

Inventor(s) Michael N. Gilano It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, line #2: cancel propoargylphthaliml.de" and substitute --propargylphthalimide-- therefor.

Column 8, line 28: cancel R"R' and substitute R"X' therefor.

Signed and sealed this 23rd day of July 1971 (SEAL) Attest:

0'. MARSHALL DANN Commissioner of Patents McCOY M. GIBSON, JR. Attesting Officer 

2. The electroless plating bath of claim 1 wherein from 0.0001 to 0.001 mole, per mole of cupric ion, of a secondary stabilizer having the general formula: R''''X''(CH2)n-C*CH wherein R'''' is an alkyl group having 1 to 12 carbon atoms, a phenyl or naphthyl group, or a substituted Phenyl or naphthyl group; X'' is a thio, a sulfonyl, a sulfoxide, oxy, carbonyl or imino group; n is 1, 2 or 3; or R''''X'' is an imide or a heterocyclic ring composed of nitrogen and carbon with or without oxygen group, is also maintained in the bath.
 3. The electroless copper plating bath of claim 1 wherein X and Y are oxygen and R is an alkyl group of 1 to 4 carbon atoms.
 4. The electroless copper plating bath of claim 1 wherein the primary stabilizer is diethyl ester of O,O-dimethyldithiophosphoryl succinic acid or diethyl p-nitrophenyl thionophosphate.
 5. The electroless copper plating bath of claim 1 wherein ZR'' is a mercapto group.
 6. The electroless copper plating bath of claim 1 wherein the primary stabilizer is a dialkylmercaptothionophosphate.
 7. The electroless copper plating bath of claim 6 wherein the dialkyl group is dimethyl, diethyl or di-n-propyl.
 8. The electroless copper plating bath of claim 2 wherein n is 1 and R''''X'' is a phthalimide group.
 9. The electroless copper plating bath of claim 2 wherein the bath contains diethyl ester of O,O-dimethyldithiophosphoryl succinic acid and N-propoargylphthalimide or N-propargyloxyphthalimide.
 10. In a process of electroless copper plating which includes immersing a receptive surface to be plated in an alkaline aqueous bath having a pH in the range 10.5 to 14, and comprising water, a water soluble copper salt, a complexing agent for cupric ion, and formaldehyde, the improvement of maintaining in the bath 0.0001 to 0.001 mole, per mole of cupric ion, of a primary stabilizer having the general formula:
 11. The process of claim 10 wherein from 0.0001 to 0.001 mole, per mole of cupric ion, of a secondary stabilizer having the general formula: R''''X''(CH2)n-C*CH wherein R'''' is an alkyl group having 1 to 12 carbon atoms, a phenyl or a naphthyl group, or a substituted phenyl or naphthyl group; X'' is a thio, a sulfonyl, a sulfoxide, oxy, carbonyl or imino group, n is 1, 2 or 3; or R''''R'' is an imide or a heterocyclic ring composed of nitrogen and carbon with or without oxygen group, is also maintained in the bath.
 12. The process of claim 10 wherein X and Y are oxygen and R is an alkyl group of 1 to 4 carbon atoms.
 13. The process of claim 10 wherein the primary stabilizer is diethyl ester of O,O-dimethyldithiophosphoryl succinic acid or diethyl p-nitrophenyl thionophosphate.
 14. The process of claim 10 wherein ZR'' is a mercapto group.
 15. The process of claim 10 wherein the primary stabilizer is a dialkylmercaptothionophosphate.
 16. The process of claim 15 wherein the dialkyl group is dimethyl, diethyl or di-n-propyl.
 17. The process of claim 11 wherein n is 1 and R''''X'' is a phthalimide group.
 18. The process of claim 11 wherein the bath contains small amounts of diethyl ester of O,O-dimethyldithiophosphoryl succinic acid and N-propargylphthalimide or N-propargyloxyphthalimide. 